In Recent years, with the view point of increasing the integration degree and the operation speed of semiconductor devices typically represented by a large scale integrated circuits (LSI), reduction of the inter-layer capacitance has been required. Then, studies have been conducted vigorously for introducing low dielectric constant films with lower relative dielectric constant than that of existent silicon oxide films (relative dielectric constant of from 3.9 to 4.2) as the inter-layer insulating films.
Typical examples of the low dielectric constant film with the relative dielectric constant of 3 or less include organic siloxane films, inorganic siloxane films, and organic aromatic polymer films. The organic siloxane film mainly comprises silicon, oxygen, carbon and hydrogen and has a siloxane skeleton and methyl group (—CH3). Typical examples are methylated siloxane (MSQ) and methylated hydrosiloxane (HMSQ) films. The inorganic siloxane film mainly contains silicon, oxygen, and hydrogen and has a siloxane skeleton and a hydrogen group (—H). A hydrosiloxane (HSQ) film is a typical example. The aromatic organic polymer film mainly comprises carbon and hydrogen (in some cases, also contains a micro-amount of silicon and oxygen), and has a benzene ring skeleton. Typical examples are SiLK (name of products) manufactured by Dow Chemical Co. and CYCLOTENE (name of products) also manufactured by Dow Chemical Co., FLARE (name of products) manufactured by Honeywell Co., and CRA (name of products) manufactured by Sumitomo Bakelite Co.
Further, while the relative dielectric constant of the low dielectric constant films described above is from 2.6 to 3.0, porous low dielectric constant films (relative dielectric constant of 2.5 or less) with the relative dielectric constant being further lowered by introducing voids in the low dielectric constant films have been studied extensively. According to Non-Patent Document 1, a low dielectric constant film with the relative dielectric constant of 2.4 or lower is necessary for LSI in the 65 nm generation. Further, a low electric constant film with the relative dielectric constant of 2.1 or lower is necessary for LSI in the 45 nm generation.
A most prospective candidate for the porous low dielectric constant films described above is a porous organic siloxane film. The porous organic siloxane film can have various combinations of composition and dielectric constant. As the index for expressing the composition, use of a carbon content/silicon content (elemental ratio of the carbon content to silicon content) is convenient.
Patent Document 1 discloses a method of manufacturing a coating type organic siloxane film. As the raw material for forming the siloxane skeleton, tetraethoxy silane of 1.2 g and methyltrimethoxy silane of 6.3 g are used. Further, as a thermally decomposing material for forming voids in the film, polyethylene glycol monomethacrylate of 5.2 g is used. The raw materials and a solvent are mixed to prepare an organic siloxane coating solution and a coating type organic siloxane film is obtained by rotary coating and thermosetting. The carbon content/silicon content (can be calculated based on the molar ratio of tetraethoxy silane and methyltrimethoxy silane) of the organic siloxane film after thermosetting is 0.89 and the relative dielectric constant is 2.0. Further, Non-Patent Document 2 discloses the carbon content/silicon content and the relative dielectric constant of the coating type organic siloxane film. It is described that the carbon content/silicon content is 0.60 (result of analysis by X-ray photoelectron spectroscopy) and the relative dielectric constant thereof is 1.97 for the organic siloxane film shown as an example.
Further, Patent Document 2 discloses a method of manufacturing a plasma type organic siloxane film. After introducing diethoxymethyl silane as the raw material for forming the siloxane sleketon and α-terpen oil as a void-forming thermally decomposing material in the film into a vacuum chamber and reacting them through polymerization by a plasma chemical vapor deposition method, α-terpen oil is removed by annealing. The carbon content/silicon content is 0.69 to 0.75 and the relative dielectric constant is 2.10 to 2.80 for the plasma type organic siloxane film obtained as described above (result of analysis by X-ray photoelectron spectroscopy).
In addition to the literatures shown above, various techniques concerning the porous organic siloxane film having relative dielectric constant of 2.5 or less have been reported. Further, various preparation examples of semiconductor devices using the porous organic siloxane film as the inter-layer insulating film have also been reported (for example, in Patent Document 3).
On the other hand, there has been reported a concern about the reliability of the porous organic siloxane film and the semiconductor device using the same as an inter-layer insulating film. According to Non-Patent Document 3, it has been pointed out that the relative dielectric specific constant increases due to electric field stress in the case where the low dielectric constant film is used continuously for a long time as the inter-layer insulating film. That is, it suggests that the relative dielectric constant of the low dielectric constant film has a limited life. In the examples shown in the literatures, some porous organic siloxane films (relative dielectric constant of 2.3) have a relative dielectric constant life of about 100 years under actual conditions of use. Further, it is described that some porous organic siloxane films (relative dielectric constant of 2.4) have a relative dielectric constant life of only about 30 days under the actual use conditions. In the case of using a porous organic siloxane film having the relative dielectric constant life of only about 30 days as the inter-layer insulating film, it is anticipated also the life for the inter-wiring capacitance of the semiconductor device is about only 30 days. However, a performance for 10 years or more has to be generally ensured for semiconductor devices, and the porous organic siloxane film with such a short relative dielectric constant life is not suitable to the inter-layer insulating film.    Patent Document 1: JP-A No. 2001-122611    Patent Document 2: JP-A No. 2004-6822    Patent Document 3: JP-A No. 2003-243384    Non-Patent Document 1: International Technology Roadmap for Semiconductors 2003 Edition, International Roadmap Committee, 2003    Non-Patent Document 2: Technical Digest of IEEE International Devices Meeting, 2003, p. 35.5.1-35.5.4    Non-Patent Document 3: Journal of The Electrochemical Society, vol. 150, No. 12, 2003, p. F203-F205.